This invention relates to injecting a steam-foam-forming mixture into a subterranean reservoir to displace oil. More particularly, the invention relates to improving such an oil displacement by using a mixture containing only as much noncondensible gas as required to reduce substantially the mobility of a steam-foam-forming mixture containing a given kind and amount of surfactant and electrolyte.
Steam-foam-forming mixtures have been described in a number of patents. For example, U.S. Pat. No. 3,707,193 by T. K. Smith, Y. C. Chiu and J. B. Lawson, describes a well cleaning process involving injecting a steam-foam-forming mixture of wet steam, noncondensible gas, surfactant and electrolyte. U.S. Pat. No. 4,086,964 by R. E. Dilgren, G. J. Hirasaki, D. G. Whitten and H. J. Hill, describes a steam drive oil recovery process involving injecting a steam-foam-forming mixture of wet steam, noncondensible gas, surfactant and electrolyte. U.S. Pat. No. 4,161,217 by R. E. Dilgren and K. B. Owens describes a similar process involving injecting both the steam-foam-forming mixture and a hot water and noncondensible gas foam which tends to underride such a steam-containing mixture. U.S. Pat. No. 4,393,937 by R. E. Dilgren and K. B. Owens describes a process for displacing oil by injecting a steam-foam-forming mixture of wet steam, noncondensible gas, electrolyte and an alpha-olefin surfactant. Commonly assigned copending patent application Ser. No. 530,156 filed Sept. 7, 1983 by P. B. Ritter and R. E. Dilgren describes a steam soak oil recovery process involving injecting a steam-foam-forming mixture of wet steam, noncondensible gas, electrolyte and a surfactant that forms a foam which is more mobile in reservoir pores which contain oil than in those which do not.
Thus, it is known, from descriptions such as those identified above, that, in a steam-foam-forming mixture capable of forming a foam or mixture of steam, gas and water which is significantly less mobile than steam alone, the presence of at least some noncondensible gas is desirable. However, the noncondensible gas component of such a mixture is a relatively expensive ingredient that tends to increase the cost of using such a mixture. And, it was not previously known how to determine how much noncondensible gas was required for a given reservoir situation. It was known to be desirable to avoid using more of the noncondensible gas than necessary, but field test results indicated that, at least in some reservoirs, some noncondensible gas was essential. Moreover, it was found that the amount of noncondensible gas required was apt to be too small to be reliably measured by means of laboratory testing.
In view of this, the present invention is, at least in part, premised on a discovery that relatively reliable estimates of the amount of noncondensible gas which is actually needed in a particular reservoir situation can be made by relatively simple calculations which are based on reservoir properties that can be measured or determined by conventional procedures.